As a semiconductor becomes finer, an amount of voltage supplied to a semiconductor apparatus decreases, recently. An instrument tends to employ a semiconductor apparatus for the purpose of reducing power consumption. Further, a difference between input and output voltages becomes smaller in order to improve efficiency of a power supply circuit that supplies a power source to a semiconductor apparatus. As shown in FIG. 5, an error amplifier circuit 101 of a conventional constant voltage circuit 100 amplifies a difference between a division voltage Vfb, which is obtained by dividing an output voltage Vo with resistances R101 and R102, and a reference voltage Vref. The error amplifier circuit 101 then controls a gate voltage of an output transistor M101 connected to an output terminal OUT so that the output voltage Vo becomes a prescribed level.
In order to improve efficiency of a constant voltage circuit with the above-mentioned configuration, it is important to decrease a difference between the input and output voltages Vi and Vo as small as possible, and thereby reducing power consumption in the output transistor M101. A difference between input and output voltages Vi and Vo is needed to be more than a product of a turn on resistance and output current of the output transistor M101. When the turn on resistance of the output transistor M101 is large, the difference between the input and output voltages Vi and Vo cannot be decreased. Further, when the voltage supplied to the Semiconductor apparatus decreases, and the input voltage Vi decreases down to approximately a threshold voltage for the output transistor M101 as mentioned above, the output transistor M101 cannot be sufficiently turned on, and thereby the turn on resistance of the output transistor M101 becomes large. Then, in order to decrease the turn on resistance of the output transistor M101, an area of an element of the output transistor M101 is increased or a transistor having a low threshold voltage is utilized.
FIG. 6 illustrates another conventional constant voltage circuit discussed in Japanese Patent Application No. 03-204012, wherein the same numeral numbers and marks represent the same and corresponding parts as in FIG. 5. A constant voltage circuit 100a of FIG. 6 includes an output transistor M111 of a source follower connection type using a NMOS transistor, an error amplifier circuit 101, a reference voltage generation circuit 102, a charge pump circuit 103, and a pair of output detection use resistance R101 and R102.
As shown, when a difference between input and output voltages Vi and Vo is small and is less than a threshold voltage for the output transistor M111, the output transistor M111 cannot be turned on. Then, the charge pump circuit creates voltage larger than the input voltage Vi and supplies it to the error amplifier circuit 101 as a power source. Thus, the error amplifier circuit 101 is able to output voltage larger than the input voltage Vi and drive the output transistor M111 even if the difference between the input and output voltages is less than the threshold voltage for the output transistor M111.
However, since a ratio of an area occupied by the output transistor M101 is significantly large in such a constant voltage circuit of FIG. 5, a chip size becomes large when integrated while an element size of the output transistor M101 is increased. Further, an input capacity of a gate of the output transistor M101 also increases, and a high-speed response is not achieved. Further, a MOS transistor having a low threshold voltage usually has a large leak current when turned off sometimes resulting in a problem. Further, since the output transistor M111 includes source follower configuration in the circuit of FIG. 6, an amplification rate is less than one at the output transistor M111, a performance, such as ribble? removal rate, etc., is deteriorated.